A 27-year-old Cornell grad tragically landed in the Park Avenue killer’s crosshairs — because he accidentally took the wrong elevator to her employer’s floor and encountered her, shooting her dead.

The gunman, who said in a note he believed he had CTE, targeted skyscraper that houses the NFL's headquarters.

Wearing body armor and wielding an assault-style rifle, the alleged gunman burst into a Midtown Manhattan office building shooting victims at random.

Shane Tamura, the Park Ave. gunman who killed an NYPD officer and three others before taking his own life, left behind a suicide note saying he suffered from CTE, a brain injury often linked to playing football, police sources said Tuesday.

The guard had been fatally shot as he crouched behind a desk to take cover as the gunman sprayed the building’s lobby with bullets, police said.

The assault rifle used in mass shooter Shane Tamura’s deadly rampage at a NYC skyscraper was assembled piece by piece – with the critical part purchased by his boss, according to officials and sources.